In plain words: A test feeds a model a book's first few words and measures how much text it reproduces, showing how much of that book it memorized. Across 200 books and 14 models, most were barely memorized, but one model reproduced Harry Potter almost whole.
Abstract · Extracting memorized pieces of (copyrighted) books from open-weight language models
Plaintiffs and defendants in copyright lawsuits over generative AI often make sweeping, opposing claims about the extent to which large language models (LLMs) memorize protected expression from books in their training data. We show that these polarized positions dramatically oversimplify the relationship between memorization and copyright. To do so, we develop a technique to measure memorization of books, which we apply to 200 books and 14 open-weight LLMs. Through over 3000 experiments, we show that memorization varies both by model and book. With respect to our specific extraction methodology, we find that most LLMs do not memorize most books -- either in whole or in part; however, there are notable exceptions. For instance, Llama 3.1 70B entirely memorizes some books, like Harry Potter and the Sorcerer's Stone; memorization is so extensive that one can deterministically extract the whole book almost verbatim using the book's first few words as an initial prompt. We discuss why our results have significant implications for copyright cases, though not ones that unambiguously favor either side.
A. Feder Cooper, Mark A. Lemley, Allison Casasola, Ahmed Ahmed, Aaron Gokaslan, Amy B. Cyphert, Christopher De Sa, Daniel E. Ho, Percy Liang
arXiv:2505.12546 · cs.CL, cs.CY, cs.LG · submitted May 18, 2025 · updated Jul 20, 2026
abstract · pdf · COLM 2026
I don't think anyone denies that frontier models were trained on copyrighted material - it's well documented and public knowledge. (and a separate legal question regarding fair-use and acquisition)
I also don't think anyone denies that a model that strongly fits the training data approximates the copy-paste function. (Or at the very least, if A then B, consistently)
In practice, training resembles lossy compression of the data. Technically one could frame an LLM as a database of compressed training inputs.
This paper argues and demonstrates that "extraction is evidence of memorization" which affirms the above.
In terms of LLM output (the valuable product customers are paying for) this is familiar, albeit grey, legal territory.
https://en.wikipedia.org/wiki/Substantial_similarity
When a customer pays for an AI service, they're paying for access to a database of compressed training data - the additional layers of indirection sometimes produce novel output, and many times do not.
Unless you advocate for discarding the whole regime of intellectual property or you can argue for a better model of IP laws, the question stands: why shouldn't LLM services trained on copyrighted material be held responsible when their product violates "substantial similarity" of said copyrighted works? Why should failure to do so be immune from legal action?